1. Field of the Invention
This invention relates to the inhibition of angiogenesis, and more particularly to the use of histidine-rich glycoprotein protein (HRGP) as an inhibitor of angiogenesis.
2. Description of the Related Art
HRGP is a heparin-binding plasma protein identified by Heimburger at al. See, Heimburger et al. (1972) Physiol. Chem. 353:1133-1140. The average concentration of HRGP in plasma is around 100 μg/ml. See, Drasin and Sahud (1996) Thrombosis Research 84:179-188. The amino acid sequences of mouse, rat, (Hulett and Parish (2000) Immunology and Cell Biology 78: 280-287), rabbit (Borza et al. (1996) Biochemistry 35:1925-1934) and human (Koide et al. (1986) Biochemistry 25:2220-2225) HRGP have been resolved. Structurally, the HRGP molecule can be divided into three main domains. The amino-terminal domain, encompassing about 250 amino acid residues, contains two cysteine protease inhibitor (cystatin)-like stretches, which allows the classification of HRGP as a member of a cystatin superfamily together with α2HS glycoprotein and kininogen. There are six putative sites for N-linked glycosylation in the amino terminal portion of HRGP. A central domain contains tandem repeats of the pentapeptide [H/P]-[H/P]PHG. Both the central domain and the 105 amino acid C-terminal domain are di-sulfide bonded to the cystatin-like stretches in the amino-terminal domain (Borza et al., 1996). HRGP binds heparin/heparan sulfates (Lijnen et al. (1983) J. Biol. Chem. 258:3803-3808) in a pH-dependent interaction (Peterson et al. (1987) J. Biol. Chem. 262: 7567-7574). The isolated histidine-proline-rich middle domain mediates heparin binding (Borza et al., 1996), but the amino terminal domain has also been implicated in heparin binding. See, Koide et al. (1986) FEBS Lett. 194:242-244. A congenital deficiency of HRGP has been mapped to a single-nucleotide mutation, which results in replacement of Gly85 to Glu in HRGP. This mutation leads to inefficient processing of the protein, the majority of which is retained intracellularly. As a consequence, the serum levels of HRGP are reduced to 25-30% of normal levels. See, Shigekiyo et al. (1993) Thromb. Haemost. 70:263-265 and Shigekiyo et al. (1998) Blood 91:128-133.
Angiogenesis, the process of generating new blood vessels leading to neovascularization, is essential during embryonic development, ovulation, cyclical development of the uterine endometrium, wound healing, tissue and organ growth, inflammation and wound healing. Unbalanced neovascularization is believed to contribute to the pathogenesis of certain disease states, such as arthritis, psoriasis, hemangiomas, diabetic retinopathy and retrolental fibroplasia, and to allow tumor growth and metastasis to occur. See, e.g., U.S. Pat. No. 5,854,205. Tumor cells must attract new vessels to expand locally and produce metastasis.